一、訊號量引出

當我們編寫的程式使用執行緒的時候，總是有一部分臨街程式碼，需要確保只有一個程序(或一個執行緒)可以進入這個臨街程式碼並擁有對資源的獨佔式訪問
——》檔案鎖，提供了一個原子化的檔案建立方法，它允許一個程序通過一個令牌(新建立的檔案)來取得成功，這個方法比較適合於處理簡單的問題

二、訊號量定義

三、訊號量函式

1、建立一個新的訊號量或取得一個已有的訊號量

2、改變訊號量的值

3、控制訊號量

4、訊號量例子
$:./sem1 & ./sem1 1
同時執行兩個程序，搶佔操作op_char 資源，輸出的OX應是均勻的

/* After the #includes, the function prototypes and the global variable, we come to the
 main function. There the semaphore is created with a call to semget, which returns the
 semaphore ID. If the program is the first to be called (i.e. it's called with a parameter
 and argc > 1), a call is made to set_semvalue to initialize the semaphore and op_char is
 set to X. */

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/sem.h>

#if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)
    /* union semun is defined by including <sys/sem.h> */
#else
    /* according to X/OPEN we have to define it ourselves 操作結構體*/
    union semun {
        int val;                    /* value for SETVAL */
        struct semid_ds *buf;       /* buffer for IPC_STAT, IPC_SET */
        unsigned short int *array;  /* array for GETALL, SETALL */
        struct seminfo *__buf;      /* buffer for IPC_INFO */
    };
#endif
static int set_semvalue(void);
static void del_semvalue(void);
static int semaphore_p(void);
static int semaphore_v(void);
static int sem_id;

int main(int argc, char *argv[])
{
    int i;
    int pause_time;
    char op_char = 'O';

    srand((unsigned int)getpid());//根據PID生成隨機數
    
    sem_id = semget((key_t)1234, 1, 0666 | IPC_CREAT);//初始化訊號量，IPC_CREAT標誌作用是訊號量不存在的時候建立，存在的時候也不會報錯

    if (argc > 1) {
        if (!set_semvalue()) {
            fprintf(stderr, "Failed to initialize semaphore\n");
            exit(EXIT_FAILURE);
        }
        op_char = 'X';
        sleep(2);
    }

/* Then we have a loop which enters and leaves the critical section ten times.
 There, we first make a call to semaphore_p which sets the semaphore to wait, as
 this program is about to enter the critical section. */

    for(i = 0; i < 10; i++) {        

        if (!semaphore_p()) exit(EXIT_FAILURE);
        printf("%c", op_char);
        fflush(stdout);//清空標準輸出的快取
        pause_time = rand() % 3;
        sleep(pause_time);//等待隨機時間
        printf("%c", op_char);
        fflush(stdout);

/* After the critical section, we call semaphore_v, setting the semaphore available,
 before going through the for loop again after a random wait. After the loop, the call
 to del_semvalue is made to clean up the code. */

        if (!semaphore_v()) exit(EXIT_FAILURE);
        
        pause_time = rand() % 2;
        sleep(pause_time);
    }    

    printf("\n%d - finished\n", getpid());

    if (argc > 1) {    
        sleep(10);
        del_semvalue();
    }
        
    exit(EXIT_SUCCESS);
}

/* The function set_semvalue initializes the semaphore using the SETVAL command in a
 semctl call. We need to do this before we can use the semaphore. */

static int set_semvalue(void)
{
    union semun sem_union;

    sem_union.val = 1;
    if (semctl(sem_id, 0, SETVAL, sem_union) == -1) return(0);//規範訊號量的動作，SETVAL用來把訊號量設定為一個已知的值
    return(1);
}

/* The del_semvalue function has almost the same form, except the call to semctl uses
 the command IPC_RMID to remove the semaphore's ID. */

static void del_semvalue(void)
{
    union semun sem_union;
    
    if (semctl(sem_id, 0, IPC_RMID, sem_union) == -1)
        fprintf(stderr, "Failed to delete semaphore\n");
}

/* semaphore_p changes the semaphore by -1 (waiting). */

static int semaphore_p(void)
{
    struct sembuf sem_b;
    
    sem_b.sem_num = 0;
    sem_b.sem_op = -1; /* P() */
    sem_b.sem_flg = SEM_UNDO;
    if (semop(sem_id, &sem_b, 1) == -1) {
        fprintf(stderr, "semaphore_p failed\n");
        return(0);
    }
    return(1);
}

/* semaphore_v is similar except for setting the sem_op part of the sembuf structure to 1,
 so that the semaphore becomes available. */

static int semaphore_v(void)
{
    struct sembuf sem_b;
    
    sem_b.sem_num = 0;
    sem_b.sem_op = 1; /* V() */
    sem_b.sem_flg = SEM_UNDO;
    if (semop(sem_id, &sem_b, 1) == -1) {
        fprintf(stderr, "semaphore_v failed\n");
        return(0);
    }
    return(1);
}